1. Field of the invention
The present invention relates to a group III nitride compound semiconductor laser. Especially, the present invention relates to a group III nitride compound semiconductor laser having a resonator part of ridge type.
2. Description of the Related Art
A semiconductor laser which has a cladding layer and an active layer made of a group III nitride compound semiconductor (AlxGayIn1-x-yN, 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61 and 0xe2x89xa6x+yxe2x89xa61) has been known. The conventional semiconductor laser is a semiconductor diode having multi-layer structure with multiple group III nitride compound semiconductor layers, which are formed on a sapphire substrate in sequence. A conventional example of the semiconductor diode is shown in FIG. 5.
A semiconductor laser 900 shown in FIG. 5 has the following eight layers formed on a sapphire substrate 91 in sequence: a buffer layer 92; an n-layer 93; an n-cladding layer 94; an n-guide layer 95; an active layer 96 made of multiple quantum well (MQW) layer; a p-guide layer 97; a p-cladding layer 98; and a p-contact layer 99. As shown in FIG. 5, a resonator part (a ridged resonator part) A is formed by using, e.g., photoresist and etching, and a positive electrode 901 and a negative electrode 902 are formed on the upper surface of the p-contact layer 99 and the etched surface of the n-layer 93, respectively.
The active layer 96, comprising a multiple quantum well (MQW) layer, is a semiconductor layer which functions as a main layer to oscillate laser. Each carriers (holes and electrons) injected from the positive electrode 901 and the negative electrode 902 combine in the active layer 96, that causes laser oscillation. The n-guide layer 95 and the p-guide layer 97 cannot prevent carriers from injecting into the active layer 96, but function to confine laser light into the active layer 96 by using refractive index difference between the active layer 96 and each of the n-guide layer 95 and the p-guide layer 97. Also, the n-cladding layer 94 and the p-cladding layer 98 function as electric potential barrier, so that they can accumulate injected carriers (holes and electrons) in the n-guide layer 95, the active layer 96 and the p-guide layer 97. And the n-layer 93 and the p-contact layer 99 are semiconductor layers which are formed in order that carriers can be injected smoothly from the negative electrode 902 and the positive electrode 901 to the layers existing between the n-cladding layer 94 and the p-cladding layer 98, respectively.
In order that the semiconductor laser made of group III nitride compound semiconductor can oscillate laser efficiently, the cross-section of electric current path of the semiconductor laser is, for example, narrowed by decreasing the contact area of electrodes, or by decreasing the width w of the positive electrode 901. And in order to narrow the cross-section of the electric current path, the p-cladding layer 98 is etched to about half a thickness of the original. Also, a carrier injection part (a ridged hole injection part) B is formed by removing the upper part of the ridged resonator part A to have the same width as that of the electrode 901, as shown in FIG. 6.
However, a threshold electric current to oscillate laser cannot be decreased by forming the ridged hole injection part B. The inventors of the present invention consider that as shown in FIG. 6, this is because holes h+diffuse in a lateral direction to enlarge the width of the cross-section of the current path larger than the width of the ridged hole injection part B, i.e., the width w of the positive electrode 901, when holes h+existing in the p-cladding layer 98 conducts from the ridged hole injection part B to the ridged resonator part. The inventors of the present invention further consider that the problem persists in the current density can be solved by etching to form a deeper ridged hole injection part B.
An object of the present invention is to decrease oscillation threshold current in a ridge type of a group III nitride compound semiconductor laser. Another object of the present invention is to increase the difference of the effective refractive indices of the ridged resonator part and the other part.
To achieve the above object, a first aspect of the present invention is to obtain a group III nitride compound semiconductor laser comprising a resonator. The resonator is formed by removing multiple layers, which are made of group III nitride compound semiconductors and formed on a substrate, except the width of the resonator part. The group III nitride compound semiconductor laser comprises: an active layer which functions as a main layer to oscillate laser; a first and a second guide layers contacting to the active layer; a cladding layer which contacts to the opposite surface of the first guide layer to the active layer; a contact layer which contacts to the opposite surface of the cladding layer to the first guide layer; and an electrode formed on the contact layer. A carrier injection part is formed contacting to the resonator part by removing almost all of the contact layer and the cladding layer except the area corresponding to the width of an electrode.
The second aspect of the present invention is to obtain a group III nitride compound semiconductor laser comprising a resonator. The resonator is formed by removing multiple layers, which are made of group III nitride compound semiconductors and formed on a substrate, except the width of the resonator part. The group III nitride compound semiconductor laser comprises: an active layer which functions as a main layer to oscillate laser; a first and a second guide layers contacting to the active layer; a cladding layer which contacts to the opposite surface of the first guide layer to the active layer; a contact layer which contacts to the opposite surface of the cladding layer to the first guide layer; and an electrode formed on the contact layer. A carrier injection part is formed contacting to the resonator part by removing almost all of the contact layer and the cladding layer and at least a partial depth of the first guide layer except the area corresponding to the width of an electrode.
The third aspect of the present invention is to remove almost all of the contact layers and the cladding layers made of group III nitride compound semiconductors, formed on the side of a positive electrode. And the fourth aspect is to additionally remove till at least a partial depth of the first guide layer.
With respect to a ridge type of laser, carriers can be supplied to an active layer with improved current density by removing all of a contact layer and a cladding layer, which are formed on the side to form an electrode, except the area corresponding to the width of the electrode, and forming a carrier injection part. And carriers can be supplied to the active layer with further improved current density by removing all of the contact layer and the cladding layer, which are formed on the side to form the electrode, except the area corresponding to the width of the electrode, and by further removing at least a partial depth of a guide layer which is also formed on the side to form the electrode. A group III nitride compound semiconductor laser is etched more frequently from a positive electrode side. So current density of holes injected into the active layer can be improved effectively.